CFD simulation and smoke removal in underground garages

The basis for performing CFD simulations is the analysis of the worst fire development scenarios, representative of the given space. To prepare them correctly, not only theoretical knowledge is needed, but also solid practical foundations. How to properly perform CFD simulations? Joanna Marzec, designer in the Fire Ventilation Systems Department of MERCOR S.A. shares his knowledge and experience.

Performing CFD simulations is very important in the context of checking the correctness of the smoke removal installation design. At the facility design stage, it allows you to reduce possible design errors and predict a fire protection scenario based on forecast events.

What is CFD simulation?

CFD, or Computational Fluid Dynamics, is a branch of fluid mechanics that uses numerical methods to solve problems of fluid flows (liquids and gases). In this method, thanks to the numerical solution of partial differential equations describing the flow, it is possible to approximately determine the distribution of speed, pressure, temperature and other flow parameters. The most frequently used software in this type of studies is Fire Dynamics Simulator (FDS), which is implemented in graphical interfaces, e.g. in PyroSim.

How to prepare data for CFD simulation?

When performing a CFD simulation, first a three-dimensional model that reproduces the architecture of the analyzed space as precisely as possible should be created. Existing smoke exhaust elements, or their lack, should also be taken into account. Adding elements such as smoke curtains can significantly change the results of the fire simulation and even directly influence whether the results are positive or negative (Fig. 1.1 and 1.2).

In the next step, you should complete the initial data, including fire parameters, through, among others: introducing an appropriate fire power-time curve or parameters specific to the burned material.

 

Fig.1.1. Fire simulation results for 2 passenger cars without a drawn smoke curtain - visibility range

 


Fig.1.2 Results of fire simulation for 2 passenger cars with a smoke curtain drawn - visibility range

After entering the model, initial data, including fire parameters should be entered, through, among others: introducing an appropriate fire power-time curve or parameters specific to the burned material.

 

Fig. 2 Fire curve graph for 3 conventional cars generated from PyroSim

Why do CFD simulation?

For spaces such as garages, several goals are defined for performing simulation studies at the design stage. The main goal is to confirm the correct operation of the designed system or expose its defects and possibly improve the assumptions. Especially in the case of jet ventilation, where calculation standards are not used in design, simulations are an invaluable tool supporting the process. Another goal may be to verify the possibility of optimizing the system to minimize costs, as well as to conduct a comparative analysis of the effectiveness of different systems. However, it is important that the development, although time-consuming, is carried out with due care and that the results reflect the actual situation, without covering up the shortcomings of the analyzed solutions.

Representative scenario

Simulations should be based on verification of unfavorable fire scenarios in a given garage, which include, among others: fires located on the border of smoke zones due to the high risk of smoke in the adjacent zone, fires in close proximity to escape entrances, which may prevent safe evacuation, fires located far from exhaust points, generating smoke in large areas of the garage by dragging smoke, or fires located directly under smoke removal devices , carrying the risk of exceeding the temperatures permitted for the fans. The accuracy and reliability of the results will depend on the correct selection of scenarios, as well as parameters such as fire power, accurate model drawings and the selection of the computational mesh. Another important issue is the correct and accurate interpretation of the results themselves and the conclusions drawn on this basis, which must correlate with the results included in the calculation report.

In the case of CFD simulation, as well as during design work, the author of the study, his competences, accuracy and decisions made during the preparation of the study play a key role.

 

 

Joanna Marzec

designer from the Fire Ventilation Systems Department

MERCOR S.A.